This invention relates to a control circuit for protection devices, and more particularly to a control circuit for an automatic sectionalizing switch.
In power generation and distribution it is common to have switchgear to protect downstream equipment at various points of the distribution system. A device, such as a recloser, will open on downstream electrical faults, reclose, and if the fault is still present downstream, open again. This is repeated a number of times, usually four or five times and if the fault is still present the recloser locks open.
Downstream from the recloser on branches of the electrical distribution network are devices called sectionalizers. Sectionalizers are designed to open or disconnect sections of the distribution network downstream of the recloser on the branch having the fault. The sectionalizer will count the number of times the sectionalizer sees the fault and the number of times the current goes to zero when the recloser opens. The sectionalizer will then open on a predetermine number of counts. The sectionalizer furthest downstream from the generating source will have the smallest number of counts before it opens and remains open. This is done so that faults on the system can be isolated as far downstream from the generating source as possible. There may be as many as four sectionalizers between the recloser and a load. The sectionalizer closest to the load would open first, on one count, the next one upstream would open on two counts and so on.
A problem is encountered when the sectionalizers count as a downstream fault, an electrical fault on the upstream side of the sectionalizer. This will cause the sectionalizer to open, indicating to linemen trying to solve the problem on the distribution network, that the fault is downstream of the open sectionalizer. These up-line fault problems may be due to a number of things, one of which would be a phase to ground fault having small fault resistance. This would cause over voltage on the two unfaulted phases, which causes slightly higher current on those two unfaulted phases. If a transformer downstream saturates due to over voltage, it would draw high exciting current twice per cycle during voltage peaks, which would cause pickup on threshold sensing devices in the sectionalizer.
Another up-line problem which may cause sectionalizers to count incorrectly is that unsymmetrical fault clearing may occur at the recloser. That is, the different phases may be opened by the recloser at different times, which gives a phase imbalance in the currents, which causes the sectionalizer to count incorrectly. When one phase current goes to zero and the other phases are still energized, a large neutral, or ground, current is created.